Dynamic tracking of tumor microenvironment modulation using Kaede photoconvertible transgenic mice unveils new biological properties of viral immunotherapy

Abstract CAN-2409 is a replication-defective adenovirus that delivers the herpes simplex virus (HSV)-thymidine kinase gene to infected cells. Intratumoral administration of CAN-2409 followed by prodrug results in the formation of a toxic metabolite able to induce immunogenic cell death, exposure of tumor-associated antigens, and activation of local and systemic immune responses. We used a dynamic labeling model with MC38 tumor cells implanted in photoconvertible Kaede mice. Violet light was used to label the tumor microenvironment, distinguishing retained versus newly entering cells and allowing real-time monitoring of immune compartment changes within tumors. Administration of CAN-2409 + prodrug led to control of tumor growth and a significantly increased effector CD8+ T cell responses. Photolabeling of the tumor microenvironment (TME) revealed that rather than enhancing recruitment of T cells to the tumor, CAN-2409 altered the TME whereby newly entering and retained CD8+ T cells were significantly more proliferative. CAN-2409 supported reinvigoration of tumor associated antigen-specific CD8+ T cells and expansion of Tregs of an altered phenotype. Moreover, the combination of CAN-2409 + prodrug and anti-CTLA-4 antibody treatment further improved control of tumor growth, in part by the enhanced CD8+ T cell-mediated effector function and diminished Treg-mediated immunosuppression. Collectively, these data defined at least two temporally distinct pathways underpinning the mechanism of action of CAN-2409 action that overcome cell exhaustion and decreases immune suppression. The results also support the rationale for future clinical trials of CAN-2409 treatment combined with anti-CTLA-4 antibody therapy.